A lot of different experiments, under way in different nations have demonstrated the capability of free electron lasers (FEI,) to produce a large amount of radiation in different ranges of the spectrum (1). On the other hand each of these experiments has proved that the main limitations to the performances of an FEL come from the optical equipment used for the resonator.
The prior art for this invention includes linear output couplers for FEL oscillators, non-linear intra-cavity elements for conventional laser oscillators, and a non-linear broad band acousto-optic output coupler for FEL's.
Linear output couplers have been used for many years as components of FEL oscillators. These devices include both simple partially transmitting mirrors and intra-cavity Brewster plates tilted slightly off the Brewster angle. However, these couplers do not have the capability to modulate the FEL output pulse structure and typically permit only 1-10% of the light circulating in the optical cavity to be extracted from the resonator.
A broad variety of intra-cavity non-linear elements have also been demonstrated as components of conventional laser systems. However, the properties and problems of conventional oscillators differ substantially from the properties of the FEL. and hence the non-linear elements developed for these conventional systems either would not function as a component of an FEL, or would not yield high levels of performance. The special functions distinguishing non-linear components for FEL's from those for conventional laser oscillators include the FEL's broad-band tuneability, high peak power, and (for RF-linear or storage ring FEL's) the FEL's intrinsic mode-locked pulse. In addition, the sideband instability which limits the power output of highly saturated FEL's, is not present in conventional laser oscillators.
Finally, while a broad-band acousto-optic output coupler has been described in prior literature for FEL oscillators, this device lacked the speed necessary to extract high peak power pulses from an FEL oscillator.